1. Field of the Invention
This invention generally relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a semiconductor device such as a laser diode or the like having a step region of a mesa shape formed on a semiconductor substrate. With the fabricating method, the step region can be planarized with highly crystalline buried layers.
2. Description of the Related Art
In recent years, the optical communication techniques with the use of the laser diodes and optical fibers have achieved a remarkable breakthrough. In each home, a broadband communication is available at a low cost. With the increase of the communication traffic on a personal basis in the days to come, FTTH (Fiber To The Home) is expected to be the main stream instead of ADSL in the near future. This highly advanced optical communication techniques are requesting technical advantages and price reduction of optical semiconductor devices such as the laser diode.
FIG. 1 illustrates a structure of a low cost laser diode conventionally used for the optical communication, and is a cross-sectional view of the laser diode seen from the direction of [011]. This is disclosed in, for example, Japanese Patent Application Publication No. 2002-198616 (hereinafter referred to as Document 1). A laser diode 10 includes an n-type (100) InP substrate 11 having an n-side electrode 17 on the backside thereof. An n-type InGaAsP layer, an InGaAsP layer, a p-type InP layer, and a p-type InGaAs layer are sequentially deposited on a main plane of the n-type InP substrate 11. A step region having a post shape or mesa shape is formed by dry etching to leave a part of the laminated structure. Each layer of the step region includes an n-type InGaAsP guide layer 12, an InGaAsP-MQW active layer 13, a p-type InP clad layer 14, and a p-type InGaAs contact layer 15. On both sides of the step region, an Fe-doped highly resistant buried layers 16 are buried and planarized. The Fe-doped highly resistant buried layers 16 have been grown by metal organic chemical vapor deposition (MOCVD) with a mixed gas in which chloromethane (CH3Cl) is added to a base gas. An SiO2 protective coating 19 is provided with an opening to expose the whole plane of the p-type InGaAs contact layer 15 and a part of the Fe-doped highly resistant buried layers 16, which are adjacent to the Fe-doped highly resistant buried layers 16. Additionally, a p-side electrode 18 is provided for biasing the p-type InGaAs contact layer 15 in the opening of the SiO2 protective coating 19.
The laser diode having the structure in FIG. 1 has only two crystal growth processes with the MOCVD method. The two crystal growth processes include a formation of the laminated structure and a formation of the buried layers. The fabricating process is relatively simple and easy. Dry etching as represented by RIE (Reactive Ion Etching) is superior to wet etching with the use of a liquid etchant, with respect to controllability and reproducibility. In addition, there is an advantage in that a large area can be etched uniformly. The MOCVD method is a crystal growth process that can grow the crystal uniformly on the substrate having a large hole diameter. In addition to the above-mentioned dry etching, the MOCVD method is suitable for mass production and cost reduction of the laser diodes. Further, after the growth of the buried layers, protrusions and dents generated on the plane are etched with a compound liquid including hydrochloric acid and acetic acid to completely planarize the vicinity of the step region (the vicinity of the mesa). With the above-mentioned planarization technique, a high process yield is obtainable in the electrode formation process and thus the cost reduction becomes possible.
In order to remove the protrusions and dents generated on the plane after the growth of the buried layers and planarization of the plane, with the above-mentioned technique, it is necessary to perform the crystal growth on the buried layers to be thick in advance so that the planes of the buried layers may be higher than that of the step region. The above-mentioned crystal growth of the buried layers might increase the costs of raw materials and maintenance fees required for the crystal growth and might decrease the throughput in the production process. This will result in an increase of the production cost of the laser diode. For instance, in the case where a step region having a height of 3.0 μm is buried by buried layers having a crystal growth rate of 2.0 μm/h, it originally takes 1.5 hours for the crystal growth. However, it actually takes 1.8 hours to make the crystal grow thicker. Therefore, it is desirable that the plane state of the buried layers buried on both sides of the step region is made plane just after the crystal growth in order to eliminate the removal process of the protrusions and dents.
Conventionally, in the case where the InP step region is buried for planarization by a high-resistant InP layer, only a mixed gas of TMI (Trimethylindium) gas, a material of In and PH3 (phosphine), a material of P is supplied for the crystal growth. The InP step region extends in stripes in the direction of <0-11> on a substrate having a crystal face orientation of zincblende crystal structure (100) (for example, the InP substrate), and the crystal of the high-resistant InP layer has been grown with the MOCVD method.
Japanese Patent Application Publication No. 1-101624 (hereinafter referred to as Document 2) describes that the crystal growth is suppressed on a (111) plane of the side faces of the step region and the plane burying can be achieved, in the case where the extending direction of the step region is off at five or more degrees from the direction of <0-11>, although the crystal having the (111) plane as a facet plane is grown along the side faces of the step region with the protrusions and the plane burying cannot be achieved, in the case where the above-mentioned gas is supplied for burying the step region extending in the direction of <0-11>. Document 2 also describes that the protrusions of at least 1 μm are generated on the side faces of the step region and the flat plane is not obtainable in the case where the extending direction of the step region is deviated (off angle) from the direction of <0-11> at less than five degrees. With the above-mentioned background, it is impossible to bury the step region on a plane in the case where the step region has the off angle of less than five degrees from the direction of <0-11>, with respect to the conventional crystal growth technique that employs the mixed gas of the TMI gas and PH3 gas only as a supplied gas.
In the case where the step region extends in the direction of <011>, it is well known that excellent buried layers are obtainable by performing the crystal growth after a chlorinated organic compound based gas is added to the base gas. Japanese Patent Application Publication No. 8-97509 (hereinafter referred to as Document 3) describes that carbon tetrachloride, monochloroethane, monochloromethane, hydrogen chloride, and trichloroethane are included in the above-mentioned chlorinated organic compound based gas.
According to the experimental results made by the inventors, the inventors find that it is hard to obtain the excellent buried layers, in the case where any one of the above-mentioned chlorinated organic compound based gases disclosed in Document 3 is added to bury the step region extending in the exact direction of <0-11>. The protrusions and dents are generated because of the crystal growth of the buried layers on an A plane (In plane) of the InP (111) plane in an upper edge of the step region, a defect was found on the laminated layer on the A plane.
As described, the conventional burying techniques have difficulties in that the step region extending in the direction of <0-11> cannot be buried on a plane. This limits the flexibility of a mesa stripe orientation that can be used for fabricating the devices.